The present invention relates to electron flood apparatus for neutralising positive charge build up on a substrate during implantation of ions in the substrate by ion beam implantation apparatus.
In the manufacture of semiconductor devices, an increasingly popular processing technology is ion beam implantation whereby a beam of desired impurity ions is directed at the semiconductor substrate so that the ions become implanted in the substrate to create regions of a desired conductivity type, for example. Some regions of the substrate being implanted will be electrically insulated from the main body of the semiconductor substrate. As a result during implantation with ions, such regions become progressively charged. Usually the implanted ions are positive so that positive charge build up on the substrate surface occurs. If this charge build up exceeds the breakdown voltage of insulated layers and regions in the substrate, damage is caused.
There is thus a known requirement to neutralise charge build up on the surface of substrates during ion implantation.
Known techniques for neutralising this charge build up involve flooding the region of the ion beam immediately in front of the substrate with electrons. Any positive charge build up on a substrate tends to attract these electrons to the substrate surface to discharge the surface. Examples of electron flood apparatus, also known as plasma flood guns, are disclosed in U.S. Pat. Nos. 5,089,710 and 5,399,871. In order to be effective, these electron flood systems used for substrate neutralisation, must produce relatively low energy electrons, which can then readily be attracted to the substrate by relatively small positive charges arising on the substrate. Further, excessively energetic electrons in front of the substrate can impinge upon and xe2x80x9cstickxe2x80x9d to the substrate surface even though any positive charge on the substrate surface has already been fully discharged, and thereby build up an excess negative electrostatic charge on the surface. The degree to which such a negative electrostatic charge can accumulate on the substrate surface is related to the energy of the electrons in the region in front of the substrate.
In known ion beam implantation apparatus, the substrate, typically a semiconductor wafer, is mounted on a holder which is in turn connected to a common terminal. In the arrangements illustrated in the above two U.S. patent specifications, this substrate common terminal is connected to ground. In each of these U.S. specifications, the electron flood apparatus comprises a tube for axially receiving and passing an ion beam to the substrate. A plasma chamber has an exit aperture which communicates with an opening in the side of the tube. Inert gas is supplied to the plasma chamber. A cathode in the plasma chamber is heated from a cathode power supply to emit electrons. The plasma chamber is made electrically conducting and thereby provides an accelerating electrode. An electrical supply is connected to provide an accelerating potential between the cathode and the accelerating electrode formed by the plasma chamber. As a result, electrons emitted by the cathode are accelerated to produce a plasma in the chamber. The cathode is a directly heated filament.
In U.S. Pat. No. 5,089,710, the plasma chamber itself is either connected directly, or biased negatively relative, to the tube surrounding the ion beam in front of the substrate. As a result, electrons from the plasma in the plasma chamber entering the tube of this U.S. specification have an energy of at least the acceleration bias potential between the filament and the plasma chamber walls.
In U.S. Pat. No. ""710, the tube surrounding the beam in front of the substrate acts as a Faraday intended for measuring the total beam current impinging on the substrate. As such, the Faraday tube may be positively biased relative to the grounded substrate common terminal, since the Faraday tube must function to absorb all secondary electrons emitted by the substrate during ion bombardment. In U.S. Pat. No. ""710, the positive bias on the Faraday tube is adjustable. This positive bias has the effect of reducing the energy on the substrate of flood electrons generated in the plasma chamber of the flood source. However, the positive bias also tends to attract these electrons within the Faraday tube, thereby reducing the number of flood electrons available for charge neutralisation on the substrate. Thus, the system disclosed in U.S. Pat. No. ""710 may provide inadequate numbers of flood electrons to ensure proper neutralisation of positive charge build up on the substrate, when the energy of flood electrons at the substrate is reduced sufficiently to prevent unwanted negative charge build up on other regions of the substrate.
Referring, by comparison, to U.S. Pat. No. 5,399,871, here the filament in the plasma chamber is connected to ground, as is the support for the substrate. The tube surrounding the beam in front of the substrate is biased negatively and acts only as an electron confinement tube to encourage flood electrons emitted into the tube from the plasma chamber to diffuse to the ion beam region which generally has a small positive potential.
With such an arrangement as shown in the drawings of U.S. Pat. No. ""871, the flood electrons can have a very low energy at the substrate, typically no more than about 5 volts, if the filament supply is 5 volts. However, the current of flood electrons emitted from the plasma chamber into the tube is also relatively low and may not be sufficient in some circumstances to neutralise adequately positive charge build up on the substrate.
It is also known to use the arrangement disclosed in U.S. Pat. No. ""871 with the plasma chamber itself connected to ground. The acceleration bias between the plasma chamber and the filament, then also serves to bias the filament relative to ground. This can substantially increase the current of electrons entering the confinement tube from the plasma chamber, but also increases the energy of these electrons at the substrate, thereby increasing the risk of negative charging.
The present invention provides electron flood apparatus for neutralising positive charge build up on the substrate during implantation of ions in the substrate by ion implantation apparatus, the substrate being connected to a common terminal, the electron flood apparatus comprising a tube for axially receiving and passing an ion beam to a substrate, an opening in a side wall of the tube, a plasma chamber having an exit aperture in communication with said opening of said tube, a supply of inert gas to said plasma chamber, a cathode in said plasma chamber, a cathode power supply connected to heat said cathode to emit electrons, said plasma chamber providing an accelerating electrode within said chamber, an accelerating electrical supply connected to provide an accelerating potential between said cathode and said accelerating electrode to accelerate electrons emitted by said cathode to produce a plasma in said chamber, and a cathode bias electrical supply for connection directly between said cathode and said substrate common terminal to set a desired bias potential on said cathode relative to said substrate independently of said accelerating potential.
With this arrangement a cathode bias supply is provided in addition to the accelerating supply (also called arc supply), so that the potential of the cathode in the plasma chamber can be set relative to the substrate common terminal independently of the arc supply. This provides substantial advantages in many applications where the previously known modes of operation provided either an inadequate flux of electrons of sufficiently low energy, or an adequate flux of excessively energetic electrons. By adjusting the cathode bias supply to provide a corresponding bias potential on the cathode, a sufficient flux of electrons can be provided to prevent excess positive charging on the substrate, without producing unwanted negative charging resulting from excessively energetic electrons.
Normally, the cathode comprises a filament and said cathode power supply is a filament power supply connected directly across said filament.
The cathode bias supply (which can be regarded as a filament bias supply when the cathode is a directly heated filament) may be adjustable to set a value of said desired cathode bias potential between zero and minus fifty volts (preferably between zero and minus thirty volts).
In one embodiment, the accelerating supply is adjustable to set a value for said accelerating potential between 20 and 40 volts. The apparatus may provide in addition a tube bias electrical supply for connection between said tube and said substrate common terminal to set a desired bias potential on said tube relative to said substrate. This tube bias supply may be adjustable to set a value of said desired tube bias tube potential between zero and minus thirty volts.
Referring again to U.S. Pat. No. 5,399,871, permanent magnets are provided around the plasma chamber to provide a magnetic field in the plasma chamber substantially aligned with the axis of the exit aperture of the chamber. This field has the effect of concentrating the plasma within the plasma chamber thereby increasing the number of electrons produced in the chamber and enhancing the flow of electrons through the exit aperture. Thus the arrangement disclosed in U.S. Pat. No. ""871 includes magnet apparatus to produce a magnetic field in said plasma chamber having field lines in a central region of said chamber aligned with said exit aperture co increase the density of plasma in said chamber and promote the flow of electrons from said chamber through said exit aperture.
The present invention may also provide electron flood apparatus of the kind known from U.S. Pat. No. ""871 wherein said magnet apparatus comprises a solenoid coil surrounding said plasma chamber coaxially with said exit aperture and a solenoid power supply to energise said coil to produce said magnetic field.
Replacing the permanent magnet by a suitably positioned solenoid around the plasma chamber has surprising beneficial effects. By ensuring a magnetic flux density in the centre of the plasma chamber of at least 250 gauss and preferably at least 300 gauss, the flux of electrons from the plasma chamber into the tube surrounding the beam in front of the substrate can be increased from as low as 50 mA or less to several hundred mA without increasing the bias on the filament in the plasma chamber. As a result, by using a solenoid to produce the magnet field in the plasma chamber, a remarkable increase in electron flux is obtained without the need to increase bias voltage, and so without increasing the energy of the electrons at the substrate.